The present invention relates to a voltage converting apparatus comprising a series connection of at least four switching elements each comprising at least one semiconductor device of turn-off type and a free-wheeling diode connected in anti-parallel therewith, opposite ends of said series connection being configured to be connected to different potentials resulting in a direct voltage across said series connection and a mid point of the series connection being configured to be connected to an alternating voltage side of the apparatus while dividing the series connection in two halves, said apparatus further comprising a control unit configured to control said semiconductor devices of the switching elements for obtaining a determined alternating voltage on said alternating voltage side of the apparatus, as well as a method for converting a voltage according to the preamble of the appended independent method claim.
The invention is applicable to any voltage converting apparatus having these features and neither restricted to any levels of voltages, currents or powers to be handled by such an apparatus, although the invention is particularly directed to high voltage applications, which here means a direct voltage across said series connection exceeding 1 kV and often exceeding 50 kV. The reason for connecting at least two said switching elements and by that at least two semiconductor devices in series in each said half of said series connection is in fact to obtain high voltage switching capability, since the semiconductor devices connected in series, and also the free-wheeling diodes connected in anti-parallel therewith may then share the voltage to be taken by switching elements connected in series and belonging to the same half in a blocking state of these switching elements.
One example of a known voltage converting apparatus according to the invention in the form of a Voltage Source Converter is schematically shown in FIG. 1. Such a converter has normally three phase legs, but only one phase leg 1 is shown in FIG. 1 and has here a series connection of four switching elements 2-5 each comprising a semiconductor device 6 of turn-off type, such as an IGBT, and a free-wheeling diode 7 connected in anti-parallel therewith. Opposite ends of the series connection are connected to direct voltage poles 8, 9 on different potentials resulting in a direct voltage across the series connection. A mid point 10 of the series connection is configured to be connected to an alternating voltage side, here in the form of an alternating voltage network 11. A control unit 12 is configured to control the semiconductor devices of the switching elements for obtaining a determined alternating voltage on the alternating voltage side of the apparatus, such as direct voltage pulses according to a Pulse Width Modulation pattern or any other control method.
The number of switching elements connected in series may in the practice be much higher than shown, such as 10-50, for increasing the voltage handling capability of the converter. The control unit will when carrying out said control turn all semiconductor devices of one half on and off substantially simultaneously, so that these will in the practice act as one single switch.
Another type of voltage converting apparatus to which the present invention is directed is schematically illustrated in FIG. 2, and this is also a Voltage Source Converter, but it differs from that according to FIG. 1 by the fact that each switching element 13 has on one hand two semiconductor assemblies 14, 15 connected in series and having each a semiconductor device 16 of turn-off type and a free-wheeling diode 17 connected in anti-parallel therewith and on the other at least one energy storing capacitor 18. The control unit 19 is configured to control the semiconductor devices of said switching element to obtain two switching states, namely a first switching state and a second switching state, in which the voltage across said energy storing capacitor and a zero-voltage, respectively, is supplied across the two terminals 20, 21 of the switching element for obtaining a determined alternating voltage on the alternating voltage side 22 of the apparatus. A Voltage Source Converter of this type is disclosed for instance in DE 101 03 031 A1 and WO2007/023064 A1 and is normally called a multi-cell converter or M2LC.
A problem in common to these and other voltage converting apparatuses of the type defined in the introduction is to obtain a uniform distribution of the voltage to be taken by a series connection of switching elements on these switching elements. It is important to take measures for obtaining this for reducing transfer losses and avoiding failure of switching elements or the entire apparatus. The reason for the occurrence of different voltages across such switching elements connected in series may be charge differences in the semiconductor devices of the switching elements due to temperature difference or due to semiconductor like time differences. Furthermore, switching delay due to signal transmission, different control voltage levels of gate units controlling the semiconductor devices or different transfer characteristics of the semiconductor devices can result in switching differences and thus voltage sharing differences. Difference in passive components not shown in the Figures may be another ground for voltage miss-sharing among the switching elements.
This problem has so far been solved by using delays when switching said semiconductor devices connected in series based on information obtained from previous switching events to compensate the voltage miss-sharing. A device having too high voltage after a switching cycle must be turned off later compared to the last turn-off cycle in respect to the other semiconductor devices connected in series therewith and connected in the same string, i.e. controlled “simultaneously”. Furthermore, passive voltage sharing circuits (snubbers) are used to limit the voltage mis-sharing among the devices. The costs for such circuits in such known devices are considerable.